Gas hydrate forms when a hydrate-forming gas such as methane or any of the hydrocarbon gases, carbon dioxide, or chlorine, amongst others, is introduced into water (or where water vapor is introduced into hydrate-forming gas) to appropriate concentrations under suitable conditions of pressure and temperature and in suitable manner so that hydrate crystal nucleation and growth take place. Hydrate may also be formed when an appropriate hydrate-forming gas and water solution that is at pressures suitable for hydrate formation is chilled. Hydrate growth is not only dependent on sufficient pressures and temperature conditions; proper levels of concentration of the dissolved hydrate-forming materials (HFM's) must also be maintained.
So far as is known to me, previous attempts by others to use hydrate for seawater desalination and water treatment, which attempts introduced gas directly into the water to be treated (henceforth referred to as seawater, although other water may be treated), always ultimately resulted in the production of a slurry formed from tiny shards of relatively pure hydrate. (The shards were formed when hydrate shells formed around HFM gas bubbles would fracture.) Thus far, it has not been possible to purify such slurries sufficiently for the direct injection of HFM into seawater to be a viable process for commercially producing fresh water because too much low-salinity water had to be consumed washing the slurry.
In contrast, growth of larger masses of solid hydrate as described in U.S. Pat. No. 6,890,444, which facilitates separation of the hydrate and the residual, enhanced-salinity water, requires that HFM concentration as well as pressure and temperature in the water mass in which it is desired to nucleate and grow hydrate be maintained at appropriate levels. Published hydrate growth models and experimentation that have been described in Chapter 2 of Max et al., “Economic Geology of Natural Gas Hydrate,” Springer, Berlin, Dordrecht, 2006, demonstrate that growth of solid hydrate can be best achieved by maintaining an appropriately high concentration of HFM dissolved in water and then lowering temperature. Seawater desalination, for instance, can take place where a metered supply of dissolved HFM can be brought into the presence of hydrate in a seawater matrix, as is described in U.S. Pat. No. 6,890,444, and where pressure/temperature conditions remain suitable for hydrate growth, even where such suitable conditions are very localized.
As taught in U.S. Pat. Nos. 7,008,544 and 7,013,673, the contents of which are incorporated by reference in their entirety, gas hydrate can be induced to form in an oceanic or artificially pressurized environment in which pressure and HFM concentration are suitable for hydrate to form but in which temperature is generally too high for it to do so. In particular, within environments such as these, hydrate can be induced to form on a surface (also referred to as a “restraint”) by chilling the surface so that the pressure and temperature conditions for forming hydrate are produced locally on and near the surface. The surface may have pores or penetrations which constitute porosity of the restraint. Hydrate will grow on and outwardly away from the surface when the chilled surface is immersed in a body of water under suitable pressure and having appropriate concentrations of hydrate-forming material (HFM) dissolved therein (or in a gaseous atmosphere of HFM with appropriate concentrations of water vapor dissolved therein). Lowering temperature of the chilled surface causes hydrate to form on it and in its vicinity, thus filling the pores and blocking permeability.
In such processes, hydrate growth takes place through mass transfer of reactants from the region of hydrate instability to the narrow region of hydrate stability near the chilled porous restraint. The hydrate growth front advances into the water (or gas in the case of a gaseous atmosphere) and away from the porous restraint as water immediately at the hydrate face is cooled to the point at which hydrate is stable. Growth is sustained by the chilling of the porous restraint, which compensates for the heat of exothermic crystallization of the hydrate.
Sealing the pores of the restraint allows a pressure differential to be established and maintained across the restraint. In particular, lowering the pressure of the environment on the side of the restraint across from the hydrate (the “downstream” side) causes the hydrate closest to the porous restraint to dissociate or melt, which allows water and gas that have been contained in the solid hydrate crystal lattice to pass through the restraint into a collection region where they separate. The water derived in this process is low in salinity and is collected and concentrated for use. The process of water desalination through hydrate formation/dissociation can be steady-state, in which case hydrate growth and dissociation proceed simultaneously and at about the same rate, or cyclic, in which case there are alternating periods of predominantly hydrate growth or predominantly hydrate dissociation.